> I found out that 10k is a good value for a pull-down resistor on a
PIC-input
> but what are the limits? Is it also ok with 100k?

You really need to explain the question in a little more detail to be sure
that the answer suits your applications.
But, yes 100K is almost always OK.
1 megohm is often OK and
10 megohm is getting somewhat risky.

Assuming you are using the pin as an input which is pulled high by a
mechanical single pole switch connected to Vdd and which is pulled low by
the pull down then:

- The limits occur when input leakage either from the PIC or external
sources cause the pin to go high when the switch is open.
- Induced voltages in the input line do the same.


At the other extreme, the smallest value usable is governed by how much
current you are willing to draw through your switch contact. A 10k resistor
draws about 0.5mA and this is almost always entirely reasonable. 1K or even
100r is entirely OK but seldom necessary. Most switch contacts require a
small "wetting current" - this varies with contact material, and in
specialist cases you may wish to ensure that this current is provided by the
resistor that you use. If your switch is normally closed and the equipment
is battery operated this may be an significant addition to overall current
drain.

If you used a 10M resistor then an induced voltage in a long input lead
might cause the pin to be pulled high unexpectedly. Within a piece if
equipment even 10M is probably safe. Leakage resistance SHOULD be much
higher than 10M but for practical purposes it should not be necessary to
exceed say 1M. Exceptions might occur in special cases such as when you use
the pin as a touch switch input.



        Russell McMahon

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